Method and device for calibrating a workpiece laser-processing machine

ABSTRACT

A method and arrangement for calibrating a laser processing machine for processing workpieces. The present invention ensures a fast and reproducible method and arrangement for calibrating a laser processing machine by measuring a position of a predetermined test pattern on a test plate while taking into account any imaging errors caused by the deflection unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and an arrangement for calibrating alaser processing machine for processing workpieces.

2. Description of the Related Art

The processing of workpieces using lasers is acquiring increasingimportance principally in the field of micromachining, for example inlaser direct structuring, in laser drilling, in laser fusion, in lasersoldering, in laser inscription and in laser cutting. In laser drillingand in the structuring of wiring components (printed circuit boards andmultichip modules), it is possible to produce holes and conductorstructures having structure dimensions of less than 50 μm. Prototypeshaving conductor track structures of 20 μm have already been displayed.Accuracy is extremely important with these structure finenesses. Thegoal is to produce structure dimensions of less 10 μm. Inaccuracy insuccessive processing steps, for example, drilling and structuring onpreformed injection-molding parts or substrates, needs to be alleviated.This is because this inaccuracy to a large extent, determines thestructure's size; for example, for reliable isolation of conductortracks that run parallel, it would be necessary to take into accountlarge tolerances. This leads to large spacings between the conductortracks and thus enlarge the structure's size.

German document DE 44 37 284 discloses a method for calibrating acontroller for deflecting a laser beam, in which a light-sensitivemedium is irradiated with a laser beam at predetermined positions forthe purpose of generating a test image and afterwards, in a separateapparatus, digitized individual images of image excerpts from the testimage are generated and from them correction data are determined for thecontroller for deflecting the laser beam. The separate measurement ofthe test image is time-consuming and a separate apparatus is requiredfor the measurement; moreover, only the entire laser beam arrangement iscalibrated by means of this method, a detailed calibration of individualcomponents is not possible.

U.S. Pat. No. 4,584,455 discloses a machine for processing workpieces bymeans of laser beams, which provides, in addition to a processing laserbeam, a laser beam superposed therewith in the visible region, theposition of which on a workpiece is measured with the aid of a camera.This measured position is compared with a desired position and theprocessing laser beam is controlled in such a way that the differencebetween the desired position and the measured position is taken intoaccount. In this case, the camera only has a field of view whichencompasses part of the workpiece, with the result that deviations whichare mirrored in other parts of the workpiece cannot be taken intoaccount.

SUMMARY OF THE INVENTION

The object of the invention, therefore, is to specify a fast andreproducible method and an arrangement for calibrating a laserprocessing machine which ensures highly accurate processing ofworkpieces in the entire workpiece region.

This object is achieved according to the present invention in anarrangement for a laser processing calibration, including a lasersource, a device for shaping the laser beam, a deflection device for thelaser beam, a camera for visually observing the workpiece and anobjective for focusing a deflected laser beam on to the workpiece.

This arrangement is operated by first positioning in front of theobjective a calibration plate and recording its image. This image isgenerated by the deflection unit moving the objective and the field ofview of the camera over the calibration plate. This image is supplied toa control unit which determines any imaging errors that are caused bythe deflection unit.

Subsequently, the calibration plate is removed and a test plate isplaced in front of the objective. The laser beam then writes attestpattern onto the test plate with a predetermined drive of the deflectionunit. The position of the test pattern is measured and the opticaloffset of the laser source is determined based on the measured positionof the test pattern, taking in to account any imaging errors that werepreviously determined with patent claim 6.

The optical imaging according to the present invention, is characterizedby two mutually separate, successive method steps, first the deflectiondevice for the laser beam and the objective are measured by acalibration plate, instead of the workpiece. The calibration plate ispositioned in front of the objective and the image of the calibrationplate, is being generated by the objective and the deflection device ismeasured by a camera. The field of view of the camera is moved over thecalibration plate under the control of the deflection device.Accordingly, the entire region which can be occupied by a workpiece iscalibrated. Because the deflection device (unit) and the objectivealways remain together in the beam path, the imaging error caused bythese two units can then be analyzed in the camera, and stored, thentaken into account during the subsequent processing of workpieces. Inthe second step, the components which generate the laser beam arecharacterized. As such, the laser beam, which is generated by a lasersource and is shaped by a device for shaping the laser beam, writes apredetermined pattern (test marking) onto a test plate via thedeflection device. This test marking is subsequently measured and bytaking into account the previously determined imaging errors of thedeflection device and the objective, the optical offset that is causedby the laser source and the device for shaping the laser beam, isdetermined. Here, the optical offset is the difference between apredetermined desired position and the actual position of the testmarking. This optical offset is also stored and taken into accountduring subsequent processing of workpieces. The method operates withouta special device for measuring the test markings if the camera measuresthe test plate via the deflection device and the objective.

Another aspect of the invention is the optical construction isconfigured particularly advantageously if a mirror, transparent to thelaser light is placed in the beam path between the device for shapingthe laser beam and the deflection unit the such mirror that it reflectsthe light for the measurement of the calibration plate and/or the testplate into the camera.

The influence of workpieces on the optical imaging is advantageouslytaken into account by the camera that records the reference pointsarranged on the workpiece. Later the measured influences are stored andtaken into account during subsequent processing.

Thus, particularly in the case of structuring of non-planar areas ofworkpieces, the influence of the lack of planarity is taken into accountby the data provided beforehand, with the result that even curvedsubstrates, for example, can be written on by the laser processingmachine.

Another aspect of the invention provides for processing curvedworkpieces, in which the focal point of the imaging by the objective isadapted to the corresponding curvature of the workpiece by a device formodulating the optical path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the schematic construction of a laser processing machinefor the structuring and/or measurement of a workpiece or of a testplate,

FIG. 2 shows the schematic construction of a laser processing machinefor the measurement of a calibration plate, and

FIG. 3 shows the schematic construction of a laser processing machinefor the structuring of a workpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is explained in more detail using exemplaryembodiments as shown in FIG. 1, FIG. 2 and FIG. 3.

A laser processing machine 1 is illustrated in which a structure iswritten into a workpiece 14 by means of a laser beam 2. The structuremay be produced by drilling, fusion, soldering or cutting. The laserbeam 2 is generated by a laser source 4 and expanded by a device 5 forexpanding the laser beam 2. In order to modulate the optical path of thelaser beam 2, a device for modulation 6 is subsequently arranged in thebeam path before the laser beam is imaged onto the workpiece 14—curvedin this example—by a deflection device 7 and an objective 8. In oneembodiment, two mirrors (not illustrated), which are arrangedperpendicularly to one another are provided in the deflection device 7.The mirrors move the laser beam over the area of the workpiece 14, sothat the entire area of the workpiece 14 can be processed by thedeflected laser beam 9. Here, the device 5 expands the laser beam andthe device 6 modulates the optical path of the laser beam 2, andtogether they form a device for shaping the laser beam 2. The device 6for modulating the optical path of the laser beam 2 is controlled by acontrol device 13. Consequently, the focal point of the laser beam 2,downstream of the objective 8, is shifted in the Z-direction, i.e. inthe propagation direction of the laser beam 2. Through this control, thefocal point of the laser beam 2 can be adapted to a curved structure ofa workpiece 14. In order to measure the imaging errors of the deflectiondevice 7 and of the objective 8, a partially transparent mirror 10 isarranged in the beam path between the device 6 for modulating theoptical path and the deflection device 7. In one embodiment, this mirror10 is transparent to the laser beam 2 and reflects the light comingthrough the objective 8 and the deflection device 7 from the directionof the workpiece 14 into a camera 11, which is likewise connected to thecontrol device 13 for evaluation purposes. Another embodiment which isalso possible but is not illustrated is one in which the mirror 10 istransparent to the light from the direction of the workpiece 14 andreflects the laser beam 2 in the direction of the workpiece 14.

The method for calibrating the laser processing machine 1 proceeds, asfollows. As is illustrated in FIG. 2, first a calibration plate 3 withpatterns (for example a grid with a predetermined grid spacing) situatedon the calibration plate 3 is imaged onto the camera 11 through theobjective 8 and the deflection device 7 via the mirror 10. In this case,the control unit 13 controls the deflection device 7 in such a way thatthe beam path 12 of the image of the calibration plate 3 is moved overthe entire calibration plate 3. Using the markings of the calibrationplate 3, it is possible to characterize the beam path and thus theimaging errors of the objective 8, of the deflection device 7, of themirror 10 and of the camera 11. The imaging errors that have beenidentified are stored in the control device 13 and are compensated forduring the subsequent processing of workpieces 3.

In the second step of the method, instead of the calibration plate 3, atest plate 15, for example made of aluminum, is positioned in front ofthe objective. The laser beam 2 then writes a predetermined pattern(test marking) onto the test plate 15 by means of a predeterminedcontrol of the deflection device 7. The position of this test marking isthen imaged via the mirror 10 onto the camera 11 via the beam path inaccordance with the image of the calibration plate. In the controldevice 13, taking account of the imaging errors previously determined,the optical offset which occurred during the structuring of the testmarking is determined, stored and taken into account during theprocessing of workpieces.

In order to process curved workpieces 14, as illustrated in FIG. 3, theinfluence of the geometrical shape and/or of the optical imagingproperties of the workpieces 14 is additionally being determined. Assuch, reference points which are already present on the workpiece 14 areimaged onto the camera 11 via the objective 8 and the deflection device7 and also the mirror 10. In the control device 13. The workpieceinfluence is then determined A from the imaging errors previouslydetermined and the optical offset. The method can then subsequently beused for identical workpieces in order to control the device formodulating the optical path 6 such that that the focal point of thelaser beam 2 is oriented optimally to the surface of the workpiece 14.

This method and the arrangement also take into account, among otherthings the errors caused by the driving of the motors for the mirrors ofdeflection device 7.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

We claim:
 1. A method for calibrating a laser processing machine forprocessing workpieces, comprising: positioning a calibration plate infront of an objective; recording an image of the calibration plate,which is generated by the objective and a deflection unit, by moving theobjective and a field of view of a camera over the calibration plateunder control of the deflection unit; determining imaging errors causedby the deflection unit and by the objective from said image in a controlunit, the control unit being connected downstream of the camera; storingsaid imaging errors; positioning a test plate in front of the objective;writing a test pattern onto the test plate by a laser beam from a lasersource with a predetermined drive of the deflection unit; measuring aposition of the test pattern; determining an optical offset of the lasersource based on a measured position of the test pattern taking saidimaging errors into account; storing the optical offset; andcompensating for the stored optical offset and the stored imaging errorsduring the processing of workpieces.
 2. A method according to claim 1,further comprising the steps of: moving the field view of the cameraover the test plate by the deflection unit; and measuring the positionof the test pattern.
 3. A method according to claim 1, furthercomprising the step of: diverting the image of the calibration plateinto the camera by a mirror, said mirror being placed in a beam path ofthe laser beam between a device for shaping the laser beam and thedeflection unit, said mirror being transparent to a laser light from thelaser source, said mirror being reflective to a light with which thecamera records the image of the calibration plate.
 4. A method accordingto claim 1, further comprising the steps of: recording via the camerareference points that are arranged on one of the workpieces to beprocessed, determining in the control unit a workpiece's influence on animaging, said influence on the imagine based on said recording whiletaking into account the imaging errors and the optical offset; storingthe workpiece's influence on the imaging; and taking into account theworkpiece's influence during subsequent processing of the workpiece. 5.A method according to claim 4, for providing data for structuringnon-planar areas of workpieces, further comprising the step of: alteringan optical path of the laser beam by a device for modulating the opticalpath, said device being controlled by said control unit such that afocal point of the imaging is adapted by the objective in accordancewith a geometrical shape of the workpiece.
 6. An arrangement forcalibrating a laser processing machine for processing of workpiecescomprising: a laser source; a device for shaping a laser beam, saidlaser beam being emitted from said laser source; a deflection device forthe laser beam; a camera for visually observing a workpiece; a controldevice for the deflection device and the camera and the device forshaping the laser beam, said control device connected downstream of thecamera for imaging error determination; an objective for focusing adeflected laser beam on the workpiece; a partially transparent mirrorlocated in a path of the laser beam; a calibration plate, saidcalibration plate initially placed in front of the objective, an imageof the calibration plate is recorded by movement of a field of view ofthe camera under control of the deflection device, based on said imagethe control device determines imaging errors generated by the deflectiondevice and the objective; and a test plate, said test plate being placedin front of the objective subsequent to removal of the calibration platefrom a front of the objective, said laser beam writes a test pattern onthe test plate by a predetermined drive of the deflection unit, saidposition of the test pattern being measured and with said imagingerrors, being utilized for determination of an optical offset of saidlaser source.
 7. An arrangement according to claim 6, wherein: thedevice for shaping the laser beam further comprises a device thatexpands the laser beam and a device that modulates an optical path ofthe laser beam.
 8. An arrangement according to claim 6, wherein: thepartially transparent mirror deflects the laser beam in a direction ofthe workpiece, said partially transparent mirror deflects a light fromthe workpiece in a direction of the camera, said partially transparentmirror being placed between the device for shaping the laser beam andthe deflection device.
 9. An apparatus for compensating duringprocessing of workpieces, comprising: first means for recording an imageof a calibration plate positioned in front of an objective, the imagebeing generated by the objective and deflection unit, by moving theobjective and a field of view over the calibration plate under controlof the defection unit; second means for determining imaging errorscaused by the deflection unit and by the objective from said image;third means for storing said determined imaging errors; and laser meansfor writing a test pattern onto a test plate positioned in front of theobjective, with a predetermined drive of the deflection unit, wherein aposition of the test pattern is measured, the second means furtherdetermining an optical offset of the laser means based on the measuredposition of the test pattern, taking said determined imaging errors intoaccount; said third means storing the optical offset, wherein, duringthe processing of workpieces, the stored optical offset and imagingerrors are compensated for.
 10. An apparatus according to claim 9,further comprising: a mirror for diverting the image of the calibrationplate into the first means, said mirror being placed in a beam path ofthe laser means between a device for shaping the laser beam and thedeflection unit, said mirror being transparent to a laser light from thelaser means, said mirror being transparent to a laser light from thelaser means, and said mirror being reflective to a light with which thefirst means records the image of the calibration plate.
 11. An apparatusaccording to claim 9, wherein the first means is further for recordingreference points that are arranged on one of the workpieces to beprocessed, the second means is further for determining a workpiece'sinfluence on an imaging, said influence on the imaging based on saidrecording while taking into account the imaging errors and the opticaloffset; the third means is further for storing the workpiece's influenceon the imaging and wherein the workpiece's influence is taken intoaccount during subsequent processing of the workpiece.